U.S. Pat. No. 6,762,602 to Laursen et al. proposed a “device, e.g. an inspection pig, for inspecting conduits made from ferromagnetic materials, such as pipelines, for faults, cracks, corrosion or the like, comprising at least one pulling element, a supporting structure of variable circumference, disposed on the pulling element and comprising substantially radially disposed supporting arms each of which is pivotable about axes disposed perpendicular to the longitudinal central axis of the pulling element, and several permanent magnets disposed at the circumference of the supporting structure for generating a magnetic filed, and with sensors.” Further, Laursen, states, pertinent part: “[f]or strengthening or weakening the magnetic field generated by the permanent magnet in dependence on the circumference of the supporting structure or in dependence on the lateral separation between the permanent magnets. The permanent magnets are associated with further magnets having a magnetic field which can be varied in direction or strength. In an embodiment, the further magnets associated with the permanent magnets are permanent magnets, wherein the direction of their magnetic field can be changed by turning using an electric or mechanical actuator. In another embodiment coils are used as magnets, which can be supplied with a variable current.” See, the Abstract of U.S. Pat. No. 6,762,602 to Laursen et al.
The diametrical size of the conduit, the thickness of the conduit, and the lateral separation of the poles of the magnet are factors in the performance of the sensor. Sensors such as piezo-electric, electroacoustic, and electromagnetic sensors such as Hall, stray field and eddy current sensors may be used.
U.S. Pat. No. 6,762,602 to Laursen et al. does not effectively shunt the magnetic field produced by the poles through use of respective rotatable magnets. Additionally, the drive system of the rotatable magnets disclosed in U.S. Pat. No. 6,762,602 to Laursen et al. involves a mechanical system using springs and a toothed wheel with a selected diameter and selected number of teeth. The diameter and the number of teeth may be changed to control the rotatable magnet. Further, a spring is necessary for each of the rotatable magnets and adjustment of those magnets in a coordinated manner is difficult, and, therefore, control of the magnetic field is difficult. One of the poles in the Laursen '602 patent may be adjusted differently than the other pole because the springs and toothed gears cannot be matched or properly controlled. Spring constants and biasing of the rotatable magnets in Laursen et al. may not be equal and, hence, contribution of each of the poles to the magnetic field may be different.
Alternatively, U.S. Pat. No. 6,762,602 to Laursen et al., further discloses an electrically driven toothed wheel which is controlled by appropriate electric, electronic or mechanical sensor element for determining the lateral separation between the parallelogram supports. However, there is no disclosure in Laursen as to the coordinated operation and control of the rotatable magnets.
A more effective magnetic shunt which substantially completely reduces the magnetic field associated with a plurality of inner peripheral sensors is needed for unpiggable pipe lines. So-called unpiggable pipe lines require substantial cancellation of the magnetic field caused by permanent magnets used in conduit sensors placed in proximity to the conduit/pipe line. When the field created by the permanent magnets is substantially cancelled, the sensor may be drawn more easily through the pipeline as the sensor and its supporting structure are no longer magnetically attracted to the walls of the conduit/pipeline.
Use of a conduit sensor device requires application of north and south poles in proximity to the conduit/pipe line wall which creates a magnetic field extending into the wall of the conduit/pipe line. Control of the magnetic field which enters the conduit/pipe line is necessary for the correct interpretation of data obtained from the sensors. Sensors, such as a Hall effect sensors, measure anomalies such as cracks and deterioration of the structure of the conduit/pipeline when a magnetic field is imparted in the conduit/pipeline. Control of the magnetic field imparted in the conduit/pipeline is important as it allows correct interpretation of the data generated from the sensors. As such, a conduit sensor device which accurately controls the magnetic field in the conduit/pipeline wall is needed.